Typically, gas turbine engine combustors include combustion chambers wherein air compressed by the engine's compressor, is mixed with fuel sprayed into the combustion chamber by a fuel nozzle which extends into the combustion chamber through a hole in the combustion chamber's bulkhead. The air-fuel mixture is burned, thereby increasing the kinetic energy of the airflow through the engine to produce useful thrust. An ignitor plug which functions similarly to a common spark plug in an automobile engine, provides an electrical spark which initiates the combustion.
To maintain the proper alignment of the fuel nozzle with the various other combustion chamber components such as the ignitor plug and various air inlet apertures, as well to aid in the insertion of the nozzle into the combustion chamber for combustor assembly and maintenance, a multicomponent fuel nozzle guide which includes various cooling and combustion air apertures therein, is located in the hole in the combustion chamber wall through which the fuel nozzle extends.
It will be appreciated that the environment within a gas turbine engine combustion chamber is extremely harsh. The air fuel mixture burns in the combustion chamber at temperatures as high as 2100.degree. C. (3800.degree. F.) causing extreme thermal gradients in the combustion chamber walls. To protect the combustion chamber bulkhead from these thermal gradients, a bulkhead heat shield is disposed proximally to the bulkhead.
In order to ensure that the bulkhead heat shield does not experience thermal distress, film cooling air is guided over the bulkhead heat shield through a gap formed between the bulkhead heat shield and a radially projecting portion of the fuel nozzle guide. The spacing of this gap is maintained by means of the disposition of radially oriented ribs on a fuel nozzle guide flange. The ribs abut the bulkhead heat shield and in addition to maintaining the gap between the fuel nozzle guide and the bulkhead heat shield, serve to regulate the flow of film cooling air, and provide augmentation of the heat transfer surface area of the fuel nozzle guide.
In a typical fuel nozzle guide, there are approximately forty-eight (48) relatively thick ribs disposed about the circumference of the fuel nozzle guide flange, wherein the leading and trailing ends of the ribs are squared off. In this typical arrangement, each of the ribs has a width of approximately 0.1 inches and a radial length of 0.25 inches. Therefore, by using a known equation in the art for calculating aspect ratios (span.sup.2 /planform area), the typical rib in a fuel nozzle guide has an aspect ratio of 0.4. In addition, in the typical arrangement of these 48 ribs, each having an aspect ratio of 0.4, the leading ends of the ribs are separated from each other by a spacing of approximately 0.097 inches, while the trailing ends of the ribs are separated from each other by a spacing of approximately 0.075 inches.
A drawback in prior art fuel nozzle guides lies in the fact that these thick and squared-off ribs, combined with the spacing between the ribs, may facilitate the formation of strong vortices between film cooling jets as the film cooling flow exits the gap between the fuel nozzle guide and the bulkhead heat shield. These vortices may entrain hot gases from the combustion chamber and bring these hot gases in contact with the bulkhead heat shield, thereby possibly causing thermal distress under certain conditions.
One way to help alleviate the potential problem caused by these vortices in the prior art fuel nozzle guide is to increase cooling flow levels. However, it is known in the art that increasing cooling flow levels in the combustor upstream end increases the emissions of carbon monoxide and unburned hydrocarbons during engine operation at low power. These elevated emissions at low power are highly undesirable due to health and environmental considerations.
It is therefore an object of the present invention to provide a fuel nozzle guide that improves film cooling over the bulkhead heat shield without the need for increasing cooling flow levels.
Another object of the present invention is to provide a fuel nozzle guide that minimizes the formation of vortices between film cooling jets exiting the gap between the bulkhead heat shield and the fuel nozzle guide.
Yet another object of the present invention is to provide a fuel nozzle guide that facilitates improved cooling of the fuel nozzle guide by the cooling air.